Why does Juliet disguise her love for Romeo as a previously learned rhyme in Act 1, Scene 5 of Shakespeare's Romeo and Juliet?
My only love, sprung from my only hate!
Too early seen unknown, and known too late!
Prodigious birth of love it is to me
That I must love a loathed enemy.(150)
Juliet's closing lines for Act 1, Scene 5, do not serve to mask her love for Romeo as much as they serve to mask the embarrassment she feels at having betrayed such strong feelings for the son of an enemy and the shock she feels upon learning who Romeo really is.
Juliet has already made it very clear to her Nurse that she has fallen in love with Romeo. She made it clear when she asked her Nurse to learn Romeo's name, saying, "If he be married, / My grave is like to be my wedding bed" (142-143). Since Juliet is saying in these lines that she will likely die if she cannot marry Romeo, she is already making it very clear to her Nurse that she has developed strong feelings for Romeo.
However, we may be able to question just how seriously her Nurse has interpreted Juliet's heartfelt lines. We can see that her Nurse may not have taken Juliet very seriously when she responds to Juliet's poem with, "What's this? What's this?" Had her Nurse taken her more seriously, her Nurse most likely would have tried to counsel her out of her feelings.
Since Juliet can assume that the Nurse has not yet interpreted Juliet's professed feelings as serious, Juliet feels she is at liberty to mask how embarrassed she is about her strong feelings and the shock she feels from learning who Romeo truly is.
Juliet masks her feelings by calling her poetic declaration of love "A rhyme I learn'd even now," meaning, "a rhyme" she had just learned that night, "Of one I danc'd withal," meaning, "from someone she was unable to dance with," which refers to Romeo. These lines serve to make light of her feelings while also communicating some truth.